1. Field of the Invention
The present invention relates to a lithium ion secondary battery.
2. Description of the Related Art
In recent years, advanced hybrid cars have been expected more with a view point of environmental problems such as reduction of carbon dioxide. Power source devices such as lithium ion secondary batteries or capacitors have been developed vigorously for application to hybrid cars, etc. Improving the power density and energy density of the power source devices is an important subject in the application of mounting them on cars such as hybrid cars. Further, so-called dual mode cars capable of running only by electric power in urban areas have also been demanded in recent years and, for coping with such a demand, not only high energy density and high power density but also cycle characteristics at a higher current have been demanded as the characteristics of batteries.
Generally, for batteries used in portable equipment such as personal computers and cellular phones, while high capacity characteristics are demanded but high rate cycle characteristics are not demanded. That is, utmost about ⅓ hour rate (3 C) is demanded as load characteristics.
On the other hand, in the field of cars, an hour rate of about 1/10 (10 C), that is, a high current about three times as large as that in the battery applied for portable equipment is required and, cycle characteristics are also required. With the situations described above, it is an extremely important subject to provide a battery having high energy density, high power density and, further, high current cycle (high rate cycle) characteristics as the life time characteristics for putting the batteries to practical use in the field of hybrid cars, etc.
For attaining such a subject, negative electrode materials for lithium secondary batteries have been studied vigorously. Japanese Patent Unexamined Application No. Hei 10-36108, 2003-272627, and 2005-294011 disclose techniques of applying surface modification of a non-graphitizing carbon to the surface of graphite as the negative electrode material for the lithium secondary battery thereby improving the energy density.
The present invention intends to provide a lithium ion secondary battery excellent in the high rate cycle characteristics applicable to hybrid cars, etc. and improving the energy density and the power density.